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Pulsars and Gravity

Posted by tigerhawkvok on March 14, 2009 02:56 in astronomy , news , IYA

Seeing as I mentioned gravity just a few days ago, while the Higgs boson is appropriate fodder, this "Perspectives" piece from Science magazine is a bit more topical.

In an effort to prove ever close to the ultimate answer, physicists and astronomers have been looking to probe gravity even deeper. The best results from orbiting pulsars agree with Einstein's theory of General Relativity to within 0.2%. Alternative models predict marginally, but measurably, different results with the most relativistic systems. The ultimate measure of relativistic gravity waves depend on orbital periods, eccentricity, and masses, so by seeking systems like these we can probe gravity even more deeply than we have already.

On the face of it, it may seem that 0.2% is good enough. 99.8% accurate will, in fact, get you to the Moon (in fact, that much error on your path amounts to a 1.74 km error in your landing site on a 384399 km trip). So why does it even matter? These alternative theories posit different fundamentals about the universe, including the necessity of dark matter and cosmic origin.

Pulsar schematic, via Wikipedia

Now, the most relativistic pulsar system we know of is PSR J0737-3039 — it has an orbital period of 2.4 hours, and an orbital decay of its semimajor axis of 7 mm/yr due to gravitational radiation in the form of gravity waves. Being a pulsar, it is an incredibly accurate clock, and enables measurements of such fine precision to be made, that we need to (and can!) measure the effects of its movement through the galaxy on its orbital decay. So, Deller et al. (DOI: 10.1126/science.1167969) show that using the VLBI and about a decade of observation can smash the possible GR error (or disprove GR at these precisions!) down to 0.01%.

Think about it. We're measuring something located ~600 parsecs away (1800 light-years [Lyr], give or take), measuring its orbital decay to millimeters to narrow constraints on one of the two most accurate theories in science to being "only" 99.99% accurate, up from 99.8%. Astronomy (and science in general) — bloody amazing.

By the way — I provide many links to scientific papers, in large part for rigor's sake, but if any readers (how many readers do I have? Do I have any?) are having difficulty, I'll see what I can do about finding free alternatives, such as arXiv.org preprints.

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